﻿using System;

using NBitcoin.BouncyCastle.Math.Raw;
using NBitcoin.BouncyCastle.Utilities;

namespace NBitcoin.BouncyCastle.Math.EC.Custom.Sec
{
	internal class SecP256K1FieldElement
		: ECFieldElement
	{
		public static readonly BigInteger Q = SecP256K1Curve.q;

		protected internal readonly uint[] x;

		public SecP256K1FieldElement(BigInteger x)
		{
			if (x == null || x.SignValue < 0 || x.CompareTo(Q) >= 0)
				throw new ArgumentException("value invalid for SecP256K1FieldElement", "x");

			this.x = SecP256K1Field.FromBigInteger(x);
		}

		public SecP256K1FieldElement()
		{
			this.x = Nat256.Create();
		}

		protected internal SecP256K1FieldElement(uint[] x)
		{
			this.x = x;
		}

		public override bool IsZero
		{
			get
			{
				return Nat256.IsZero(x);
			}
		}

		public override bool IsOne
		{
			get
			{
				return Nat256.IsOne(x);
			}
		}

		public override bool TestBitZero()
		{
			return Nat256.GetBit(x, 0) == 1;
		}

		public override BigInteger ToBigInteger()
		{
			return Nat256.ToBigInteger(x);
		}

		public override string FieldName
		{
			get
			{
				return "SecP256K1Field";
			}
		}

		public override int FieldSize
		{
			get
			{
				return Q.BitLength;
			}
		}

		public override ECFieldElement Add(ECFieldElement b)
		{
			uint[] z = Nat256.Create();
			SecP256K1Field.Add(x, ((SecP256K1FieldElement)b).x, z);
			return new SecP256K1FieldElement(z);
		}

		public override ECFieldElement AddOne()
		{
			uint[] z = Nat256.Create();
			SecP256K1Field.AddOne(x, z);
			return new SecP256K1FieldElement(z);
		}

		public override ECFieldElement Subtract(ECFieldElement b)
		{
			uint[] z = Nat256.Create();
			SecP256K1Field.Subtract(x, ((SecP256K1FieldElement)b).x, z);
			return new SecP256K1FieldElement(z);
		}

		public override ECFieldElement Multiply(ECFieldElement b)
		{
			uint[] z = Nat256.Create();
			SecP256K1Field.Multiply(x, ((SecP256K1FieldElement)b).x, z);
			return new SecP256K1FieldElement(z);
		}

		public override ECFieldElement Divide(ECFieldElement b)
		{
			//return Multiply(b.Invert());
			uint[] z = Nat256.Create();
			Mod.Invert(SecP256K1Field.P, ((SecP256K1FieldElement)b).x, z);
			SecP256K1Field.Multiply(z, x, z);
			return new SecP256K1FieldElement(z);
		}

		public override ECFieldElement Negate()
		{
			uint[] z = Nat256.Create();
			SecP256K1Field.Negate(x, z);
			return new SecP256K1FieldElement(z);
		}

		public override ECFieldElement Square()
		{
			uint[] z = Nat256.Create();
			SecP256K1Field.Square(x, z);
			return new SecP256K1FieldElement(z);
		}

		public override ECFieldElement Invert()
		{
			//return new SecP256K1FieldElement(ToBigInteger().ModInverse(Q));
			uint[] z = Nat256.Create();
			Mod.Invert(SecP256K1Field.P, x, z);
			return new SecP256K1FieldElement(z);
		}

		/**
         * return a sqrt root - the routine verifies that the calculation returns the right value - if
         * none exists it returns null.
         */
		public override ECFieldElement Sqrt()
		{
			/*
             * Raise this element to the exponent 2^254 - 2^30 - 2^7 - 2^6 - 2^5 - 2^4 - 2^2
             * 
             * Breaking up the exponent's binary representation into "repunits", we get:
             * { 223 1s } { 1 0s } { 22 1s } { 4 0s } { 2 1s } { 2 0s}
             * 
             * Therefore we need an addition chain containing 2, 22, 223 (the lengths of the repunits)
             * We use: 1, [2], 3, 6, 9, 11, [22], 44, 88, 176, 220, [223]
             */

			uint[] x1 = this.x;
			if (Nat256.IsZero(x1) || Nat256.IsOne(x1))
				return this;

			uint[] x2 = Nat256.Create();
			SecP256K1Field.Square(x1, x2);
			SecP256K1Field.Multiply(x2, x1, x2);
			uint[] x3 = Nat256.Create();
			SecP256K1Field.Square(x2, x3);
			SecP256K1Field.Multiply(x3, x1, x3);
			uint[] x6 = Nat256.Create();
			SecP256K1Field.SquareN(x3, 3, x6);
			SecP256K1Field.Multiply(x6, x3, x6);
			uint[] x9 = x6;
			SecP256K1Field.SquareN(x6, 3, x9);
			SecP256K1Field.Multiply(x9, x3, x9);
			uint[] x11 = x9;
			SecP256K1Field.SquareN(x9, 2, x11);
			SecP256K1Field.Multiply(x11, x2, x11);
			uint[] x22 = Nat256.Create();
			SecP256K1Field.SquareN(x11, 11, x22);
			SecP256K1Field.Multiply(x22, x11, x22);
			uint[] x44 = x11;
			SecP256K1Field.SquareN(x22, 22, x44);
			SecP256K1Field.Multiply(x44, x22, x44);
			uint[] x88 = Nat256.Create();
			SecP256K1Field.SquareN(x44, 44, x88);
			SecP256K1Field.Multiply(x88, x44, x88);
			uint[] x176 = Nat256.Create();
			SecP256K1Field.SquareN(x88, 88, x176);
			SecP256K1Field.Multiply(x176, x88, x176);
			uint[] x220 = x88;
			SecP256K1Field.SquareN(x176, 44, x220);
			SecP256K1Field.Multiply(x220, x44, x220);
			uint[] x223 = x44;
			SecP256K1Field.SquareN(x220, 3, x223);
			SecP256K1Field.Multiply(x223, x3, x223);

			uint[] t1 = x223;
			SecP256K1Field.SquareN(t1, 23, t1);
			SecP256K1Field.Multiply(t1, x22, t1);
			SecP256K1Field.SquareN(t1, 6, t1);
			SecP256K1Field.Multiply(t1, x2, t1);
			SecP256K1Field.SquareN(t1, 2, t1);

			uint[] t2 = x2;
			SecP256K1Field.Square(t1, t2);

			return Nat256.Eq(x1, t2) ? new SecP256K1FieldElement(t1) : null;
		}

		public override bool Equals(object obj)
		{
			return Equals(obj as SecP256K1FieldElement);
		}

		public override bool Equals(ECFieldElement other)
		{
			return Equals(other as SecP256K1FieldElement);
		}

		public virtual bool Equals(SecP256K1FieldElement other)
		{
			if (this == other)
				return true;
			if (null == other)
				return false;
			return Nat256.Eq(x, other.x);
		}

		public override int GetHashCode()
		{
			return Q.GetHashCode() ^ Arrays.GetHashCode(x, 0, 8);
		}
	}
}
